The present invention relates to an aqueous ink composition suitable for use as ink for writing instruments in general use, such as ball point pen, marker, fountain pen, for recording apparatus including a pen plotter, and for ink-jet printer.
Generally, in order to perform satisfactory writing and recording by using an ink in the above-mentioned fields, it is required that, for example, the content of a dye in the ink, the density of images printed by the ink, the water resistance and light resistance of the printed images all be high, the physical characteristics of the ink composition such as viscosity and surface tension be in predetermined appropriate ranges, no precipitates be formed in the ink during the storage thereof for an extended period of time, and the pen point of writing instruments and the nozzle of ink-jet printer not be clogged with the ink when used after an extended non-use period.
However, some of the above requirements are incompatible with other requirements. For example, when the content of a dye in the ink is increased in order to increase the obtained image density, the clogging of the pen point or the nozzle is apt to occur and precipitates are easily formed during the storage of the ink.
In order to solve these problems, dyes and organic solvents which are mutually highly soluble, and additives such as surfactant have been searched. As a result, practically usable ink compositions have been developed. However, the image density obtained by such conventional ink compositions is not yet sufficiently high and the problem of the clogging of the pen point and nozzle has not yet been completely solved. Therefore it is desired to solve the clogging problem and to attain high image density at the same time.
In Japanese Laid-Open patent application No. 55-78076, for example, C.I. Direct Black 17, C.I. Direct Black 38, C.I. Direct Black 94, C.I. 27070, C.I. Direct Black 22, C.I. Direct Black 51, C.I. 15711, and C.I. Direct Black 19 are proposed as such dyes. However, their water resistance or solublity in solvents is so poor that they cannot be used satifactorily in practice even if they are used in the form of lithium salt.
Further, in Japanese Laid-Open patent application 57-202358, it is proposed that only lithium sulfonate dyes be employed, and the solvent be water and glycols. However, specific examples of such dyes given in this reference are C.I. Direct Black 19 and C.I. Direct Black 38 and their solubilities are not sufficient for use in practice.